Kinetics and mechanism of the sensitized photodegradation of lignin model compounds

The kinetics of the sensitized photodegradation of a variety of well-defined lignin model compounds was studied to determine the mechanisms responsible for lignin’s photochemically-mediated oxidation. Monomeric and dimeric models representing lignin’s phenolic end groups and nonphenolic dimers representing its inner core were studied. It was determined that the rate constants for the reaction of the deprotonated phenolic models with singlet oxygen (¹O₂) range from 0.96 to 7.2 × 10⁷ M⁻¹ s⁻¹. The models were substituted with zero, one, or two electron-donating methoxy groups on both aryl rings and, while the rate constants showed little dependence on the substitution of the nonphenolic ring, the rate constants increased dramatically with increasing methoxy substitution of the phenol. Reaction between these deprotonated models and ¹O₂ is thus proposed to occur at the phenolate ring. Under neutral conditions, it was observed that the phenolic models react with excited state sensitizer, with this reaction also occuring at the phenol ring. The sum of the rate constants for quenching of and reaction with excited state sensitizer by lignin model compound ranges from 5.4 to 75 × 10⁷ M⁻¹ s⁻¹. This study corrects previous reports that attribute the sensitized degradation of neutral lignin model compounds to reaction with ¹O₂. A nonphenolic aromatic ketone inner-core model was observed to undergo direct photolysis, and its reduced analog was not degraded by direct photolysis or reaction with ¹O₂ or excited state sensitizer. The oxidized inner-core model was also shown to be able to act as a sensitizer for the degradation of a phenolic lignin model compound.